Even more careful handling of tailcalls.

See i321 doc for description of problem and decision taken.

Author: DarkDimius

src/dotty/tools/dotc/transform/TailRec.scala

@@ -183,7 +183,7 @@ class TailRec extends MiniPhaseTransform with DenotTransformer with FullParamete
 
         val recvWiden = recv.tpe.widenDealias
 
-        val receiverIsSame = recvWiden isRef enclosingClass
+        val receiverIsSame = enclosingClass.typeRef.widenDealias =:= recvWiden
         val receiverIsSuper = (method.name eq sym) && enclosingClass.typeRef.widen <:< recvWiden
         val receiverIsThis = recv.tpe =:= thisType
 

tests/pos/tailcall/i321.scala

@@ -21,7 +21,6 @@ class i321[T >: Null <: AnyRef] {
     loop(null)
   }
 
-  @tailrec
   final def go2[U >: Null <: AnyRef](t: i321[U]): Int = t.go2(this)
 
 }